1. Field of Invention
This invention relates to a process for the suspension polymerization of vinyl chloride and, in particular, to such polymerization in which an hydrophilic polymer and a clay are used as suspending agents.
2. Brief Statement of the Prior Art
The majority of polyvinyl chloride is produced by suspension polymerization, which is a heterogeneous polymerization in which droplets of the vinyl chloride monomer are dispersed in an aqueous medium with an emulsifier and agitation. An emulsifier is desirable as it can also control the porosity of the resin product and prevent the formation of fish eyes in the product, in the manner described in U.S. Pat. No. 2,528,469. A polymerization initiator which liberates free radicals is dissolved in the monomer droplets. The suspension of the droplets is maintained by suspending agents and protective colloids. The polymerization is commonly activated by heat which releases free radicals from the initiator. Polymerization proceeds in the monomer droplets which are dispersed through the aqueous phase and, during polymerization, collisions of the droplets tend to cause coalescence of the polymer phase. The mechanical agitation establishes a dynamic equilibrium between droplet formation and coalescence. Emulsifiers are used in this suspension polymerization to stabilize the monomer droplets in the aqueous dispersion medium and to stabilize the polymer particles as they are formed and prevent their coalescence. In a typical polymerization system, the monomer droplets and polymer particles are surrounded by the emulsifier molecules which have a hydrophilic portion directed toward the dispersing aqueous phase.
As the polymerization proceeds, the viscosity of the polymerization system increases and it becomes more and more difficult to maintain dispersion of the polymer in droplets by agitation because of the viscous nature of the polymerization medium. The polymerization reaches a point where the emulsifiers, alone, cannot maintain the dispersion and cannot prevent coalescence of the polymer particles.
The tendency of the dispersion to coalesce affects the polymerization in several ways. It adversely affects the particle size distribution and shape of the polymer products. It also adversely affects the efficiency of the polymerization. When the dispersion coalesces, vinyl chloride polymer agglomerates on the interior surfaces of the reactor, including the surfaces of mixing baffles and the propellers of high speed agitation mixers. As this build up progresses, the agitation efficiency is severely affected, accelerating the coalescence of the polymer.
The polymerization is also exothermic and considerable heat is generated during the polymerization. It is common to provide indirect heat transfer with the polymerization medium to remove the heat of reaction. As the polymer droplets coalesce and coagulate, the polymer deposits on the reactor heat transfer surfaces. This reduces the efficiency of heat transfer, and adversely affects the temperature control of the process. This is an accelerating phenomenon, as the loss of temperature control results in loss of control of molecular weight and side reactions and increases the rate of deposition of the polymer on the heat transfer surfaces.
The depositing of polymer agglomerates inside the reactor also reduces the turn around time of the reaction system as the deposits must be cleaned from the reactor before it is recharged and returned to polymerization. Accordingly, it is desirable to minimize the clean out time of the polymerization reactors by minimizing the amount of polymer deposits which are formed on the reactor surfaces during the progress of the polymerization. Additionally, it is also desirable to decrease the amount of polymerization time as this too significantly affects the capacity of a polymerization plant.
Prior investigators have used protective colloids to stabilize the suspension against coalescence. Various hydrophilic polymers have been used as protective colloids in suspension polymerization of vinyl chloride. U.S. Pat. No. 3,205,204 suggests that water-soluble methyl cellulose can be used, and U.S. Pat. No. 4,360,651 suggests the use of cross-linked polyacrylic acid as suspending agents. An article reported in Chem. Abstracts 99: 3894u suggests that preferred suspending agents are polyvinyl alcohol, water-soluble gelatin and polyacrylic acid. Although hydrophilic polymers improve the polymerization somewhat, further improvements are desired.